The invention pertains generally to a control system for an electro-hydraulic actuator of the doser type and in particular to such a control system which provides improved tracking of position request signals.
Electro-hydraulic actuators of the doser type are known in the art. Such actuators operate by applying or exhausting measured quantities or "doses" of fluid to or from a fluid actuator. Each dose effects movement of the actuator in a manner similar to a stepper motor. The doses are administered in an on-off fashion by means such as solenoid actuated valves.
It is known that the dose volume can be controlled by opening a solenoid valve for a discrete time period in response to an electrical pulse of predetermined duration or width from an electronic controller. The effective output travel rate of the doser actuator can thus be varied by changing the pulse frequency and/or the pulse width with the maximum slew rate of the device being limited by the flow capacity of the solenoid valve when it remains continuously open. Such actuators are compatible with and easily controlled by modern digital electronic controls to effect a stepper motor-like response.
Doser type electro-hydraulic actuators and controls are more fully described in U.S. Pat. No. 4,256,017 in the name of James M. Eastman and assigned to the assignee of the present invention. The disclosure of Eastman is hereby incorporated by reference herein.
A control system for a doser actuator is disclosed in U.S. Pat. No. 4,366,743 issued in the name of Michael J. Leszczewski also assigned to the assignee of the present invention. The disclosure of Leszczewski is hereby incorporated by reference herein.
From these disclosures, it will be recognized that doser actuators do not have inherent digital precision comparable to stepper motors. This is because individual doses cannot be metered with the same precision as the precisely fractionally divided steps of a stepper motor. However, since most control applications utilize closed loop control systems wherein the position of the actuator is the ultimate control parameter, a doser actuator incorporated in a closed loop system can produce actuator position accuracies comparable to those of stepper motors. The doser has advantages of lower cost, reduced complexity, and higher reliability.
An example of an adaptive closed loop control system for an electro-hydraulic actuator is illustrated by U.S. Pat. No. 4,007,361, issued to Martin on Feb. 8, 1977. Such closed loop actuator systems can be used to position various components of turbine engines such as fuel control valves, exhaust nozzles 22, and variable geometry vanes. Additionally, other aircraft uses may include positional control of rotors, elevators, flaps or other components in response to pilot-initiated or automatic control system inputs.
The equilibrium condition for closed loop operation of a doser actuator requires either an error deadband for which no position correction is made or steady state limit cycling wherein the actuator continuously cycles about a desired position. Deadband operation is preferred on most applications because of reduced solenoid operation. In either case, precision of operation depends upon having a small enough minimum doser step or response to accurately move the actuator to its final position. Conversely, as the magnitude of each minimum doser step is reduced, the doser control system tends to exhibit reduced capability with respect to the accuracy of tracking a specific actuator response request. Such problems are further compounded by variations in operation between individual doser actuators, a result of manufacturing tolerances, variations in operating conditions for a specific actuator which may be caused by factors such as temperature, position, altitude, and the like, and variations in response of an actuator to what are commonly referred to as opposing and assisting loads. Accordingly, it is desirable to provide a control system for a doser actuator which is able to supply operating pulse widths for actuation of the doser solenoid valves which have a minimum value which closely approaches the minimum threshold pulse for effecting doser actuation while simultaneously providing such a control system which is able to adapt to larger doser position change requests rapidly and accurately.